Reception of frequency modulated waves and circuits therefor



R. A. HEISING Nov. 30, 1937.

RECEPTION OF FREQUENCY MODULATED WAVES AND CIRCUITS THEREFOR Filed Jan. 25, 1936 2 Sheets-Sheet 1 ABA 22 I'll l FR! UENCY 5 7 0 INVENTOR RAHEISING BY A TTORNEY FIG. 3

NOV. 30, 1937. R HElSlNG 2,100,394

RECEPTION OF FREQUENCY MODULATED WAVES AND CIRCUITS THEREFOR Filed Jan. 23, 1956 2 Shets-Sheet 2 49%; 50

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Raymond a. mum. Summit, N. 1.. assignmto- Bell Telephone Laboratories, Inco p rated. New York, N. Y., a corporation of New York Application January 2:, i936, Serial No. com 10 Claims. (01. 250-20) This inventionrelates .to electron discharge systems for selectively responding to oscillations with a definite frequency limit. More-particularly, it

relates to amplifiers of the type used for converting frequency modulated waves to amplitude modulated waves and to detecting systems for detecting frequency modulated waves.

In reception of frequency modulated waves it is customary to use a conversion unit to convert frequency modulations to amplitude modulations. This unit ordinarily takes some form of tuned circult. An object of the present invention is to provide a conversion and detection device which, in addition to its function of converting from frequency modulated waves to amplitude modulated waves, possesses such characteristics that it will not respond to oscillations above a fixed frequency or below a fixed frequency as may be desired in the particular case. 1

In detecting frequency modulated waves, it is desirable to use a balanced detector in which during non-signaling intervals any response to the unmodulated carrier wave is balanced out. This tends to eliminate both tube noise and static hiss whichwould otherwise be transmitted by the circuits as modulations imposed by the receiver upon the incoming carrier wave. An object of the invention is to provide a detecting system which shall be unresponsive to carrier waves of the normal or unmodulated frequency. h It is customary, in detection of frequency modulated waves, to convert the frequency modulations to amplitude modulations by impressing the frequency modulated waves upon a tuned circuit which yields varying amplitudes in response 0 different frequency electromotive forces according to whether the frequency approaches orrecedes from the resonance frequency of the circuit. The 'sides of the resonance curve of the usual tuned circuit are not linear and particularly toward the lower portions the curvature introduces a non-linearity which gives rise to distorquency modulated waves are impressed upon a I tuned path comprising an inductance element and a capacity element which may be connected in series. The tuned path is. resonant 'or has zero series reactance at the normal or unmodulated carrier frequency of the incoming wave. An electron discharge'detector having a cathode connected to the outer end of the inductance element and a control grid connected to the'outer end of the capacity element is effectively connected across an inductance at frequencies above normal and across a. capacity at frequencies below normal- The detector has an additional grid connected to the junction point of the inductance element and capacity element, and hence, always subject to the electromotive force impressed by the inductance element alone. It follows that for frequencies above normal both grids are subject to inductance electromotive forces and thus being in phase, permit current to pass through the discharge device. At input frequencies below normal, the grids are of opposite phase and current through the discharge device is blocked. Accordingly, .the device responds only to frequencies above normal. Its response is made proportional to the deviation from normal frequency.

In order to secure the desired balance at normal frequency, a second electron discharge detector similar to the first is also connected to the series tuned path but in opposite fashion, that is, so that its cathode is connected to the other terminal of the capacity element. For frequenciesbelow normal, both grids of the second detector will be subjected to potentials which are in phase and the detector will respond. Outputs of the two detectors are connected in push-pull relation so that response occurs from one tube as the frequency rises above normal and from the other when it falls below normal and any tendency to response at the normal frequency which might introduce tube noise or static during non-signaling intervals is balanced out.

In the drawings:

Fig. 1 illustrates a radio receiving circuit for receiving frequency modulated waves;

Figs. 2 and 3 show graphs to assist in explaining the operation of the receiving circuit of Fig. l;

Fig. 4 illustrates a modification of a portion of r the circuit of'Fig. 1;

Fig. 5 is a graph to explain the operation of the circuit of Fig. 4;

Fig. 6 shows a modified circuit of a device for converting from frequency modulated waves to amplified waves with which a separate amplitude li ildillator detector is used;

Fig. 7 is a graph to aid in explaining the operation of the circuit of Fig. 6;

Fig. 8 is a graph of'a device having a frequency amplitude conversion effect over a limited range of frequencies; and

Fig. 9 discloses a combined frequency selective and current limiting device having an operating characteristic of the type indicated by the graph of Fig. 8.

Referring to Fig. 1, a radio receiver circut is shown comprising broadly tuned antenna I and transformer 2 across the secondary winding of which is connected a path comprising variable capacity element 3 and inductance 4 which are together tuned to substantially the normal unmodulated carrier frequency of a frequency modulated wave which it is desired to receive. An electric discharge device 5 of the four-ele ment type comprises the cathode heater i having a heating current source 3 and regulating device 9, cathode l0, amplitude control grid ll, phase control grid i2 and anode [3. A similar electron discharge device 6 comprises the cathode l4, amplitude control grid l5, phase control grid l3 and anode H. A heater la in circuit with an external source l9 of heating current and regulating device 23 serves to maintain cathode l4 at a suitable operating temperature. A space current path for device 5 may be traced from cathode i through the secondary winding of transformer 2 to lead 2|, space current source 22, primary winding of audio-frequency transformer 23 and radio frequency choke coil 25 to anode l3. Similarly, the space current path for device 3 extends from cathode I4, secondary of transformer 2, lead 2|, space current source 22, primary winding of audio-frequency transformer 24, radio frequency choke ,coil 26 to anode IT. The amplitude control input terminals of device are connected to the opposite terminals of tuned path 3, 4, cathode l0 being connected to capacity element 3 and amplitude control grid ll being connected to the inductance element 4 by way of large capacity stopping condenser element 21. The amplitude control input terminals of device 6 are connected across the same tuned path 3, 4, but in reverse fashion, the oathode l4 being connected directly to inductance 4 and amplitude control grid l5 being connected through stopping capacity element 28 to the outer terminal of capacity element 3. Between cathode l3 and amplitude control grid II is a grid bias path including biasing source 29 and high resistance element 30. A corresponding biasing path including source 3| and high resistance element 32 connects cathode l4 to its associated amplitude control grid I 5. Phase control grids l2 and ii are connected through high resistances 33 and 34, respectively, to a termlnal of a grid bias source 35, the other terminal of which is connected to the junction point 36 in the tuned path 3, 4.

In accordance with the well-known method of frequency modulation, during one-half cycle of the modulating or signal frequency wave by which the carrier wave is modulated at the transmitting station, the carrier wave increases its frequency by an amount proportional at each instant throughout that half cycle to the instantaneous amplitude of the signal wave thus rising from normal carrier frequency at the beginning of a half cycle to a frequency maximum at the point of maximum signal intensity in the half cycle and then falling until it attains the normal frequency at the beginning of the next half cycle. During the. next half cycle the carrier wave will decrease in frequency from the normal frequency and will attain a minimum frequency at the peak of that signal half cycle returning again to normal carrier frequency as the signal wave returns to its zero intensity magnitude.

It will, therefore, be seen that throughout the modulation process the frequency of the frequency modulated carrier wave will exceed normal frequency during one-half cycle of the signal or modulating wave and will be less than the normal carrier frequency during the reverse half cycle of the signaling wave. At the instant of transition from one-half cycle of the signal wave to the succeeding half cycle and at all times in the absence of modulating signals, the carrier wave will have its normal frequency.

The graph 31 in the upper portion of Fig. 2 illustrates the variation of reactance between the points 33 and 39 with varying carrier frequency. At the normal carrier frequency point F0 the reactance between points 38 and 39 is zero. As the frequency increases above F'o the reactance of the path 3, 4 increases in a positive direction as illustrated. As the carrier frequency decreases from Fo the reactance increases but is of negative sign. Graphs 40 and 4| in the lower portion of Fig. 2 indicate that the effective response voltage or difierence of potential developed between points 38 and 39 is zero at the normal carrier frequency F0, but increases as the carrier frequency departs from its normal magnitude of F0 in either direction. Both response characteristics 40 and 4| are indicated as positive for a reason which wlll'presently be explained.

The normal grid bias potentials applied to grids ll, l2, l5 and it are so proportioned with respect to the anode potentials impressed on anodes l3 and I! that in the absence of incoming waves no current flows. The potential on eitherv phase control grid l2 or grid is may be represented by a graph, such as is shown in Fig. 3 in which, as indicated in full lines, the potential falls in the negative directionjn accordance with the alternating electromotive force applied between the phase control grid and its respective cathode. However, when the applied electromotive force reverses in sign during the suc ceeding'half cycle, as indicated by the dotted curve 42, there is a tendency for the phase control grid l2 to be driven positive with respect to its cathode. Accordingly, a grid leak current will traverse its circuit and a considerable portion of the driving electromotive force will be expended in the high series resistance element 33 so that the resulting potential on the grid i2 will be as indicated at 43. In other words, during its negative half cycle the phase control grid becomes highly negative, thus strongly tending to prevent any electron flow from the cathode to the anode, but duringthe positive half cycle the phase control grid becomes slightly positive so that it no longer opposes the passage of current from the cathode to the anode. It will be apparent that the electromotive force available for the amplitude control grids II and I5 is at all times less than that available for the phase control grids l2 and I6 since the two potential differences between point 36 and points 38 or 39 are each developed across a reactance of one sign, while that developed between points 38 and 39 is the difference of the two potential atoms tudes and constants involved are such that the.

phase control grids prevail and determine the through the space discharge device for the rea son that with the full negative potential impressed on the grid 12, there is too great opposition to electron flow to be overcome-by any potential which will be brought to bear upon the grid ll. Accordingly, current will flow through the electric discharge device 3. only when bo grids ii and I2 are simultaneously drivenp tive by the impressed electromotive force. It will be readily apparent that the input circuit between cathode Ill and phase control grid. I2 is connected directly across capacity element 3. If, therefore, the potential on amplitude control grid H is to be of the same phase as that upon phase control grid l2,-it can only be under .such circumstances that the input path 3, 4 across which grid ii and cathode iii are connected is capacitive as is the input path 3 of the phase conthe interval that the incoming carrier frequency is less than F0, or, in other words, during onehalf wave of the modulating signal at the transmitting station. Duringthat interval, electrons .will flow from cathode id to. anode l3 and the resultingimpulse of current will have a magnitude determined primarily by the potential of amplitude control grid ll. As indicated by graph 43 of Fig. 2, the positive potential on amplitude control grid II is a function of the departure of the carrier wave frequency from its normal frequency and increases as the carrier wave decreases in frequency. Accordingly, the discharge device 5 will transmit to the primary winding of audio-frequency transformer 23 a pulse of current of a duration corresponding to that of a half wave of the modulating or signal wave at the transmitting station and of an intensity ,proportional at each instant to the difference between the received carrier wave frequency and the normal frequency F0. Since that frequency difference is a function of the amplitude of the modulating signal, the resultant impulse supplied to the primary winding of transformer 23 will correspond in amplitude to a half cycle impulse of the modulating wave at the transmitting station.

The operation of discharge device 6 is similar in character to that of discharge device 5. In other words, electrons will not pass from cathode I4 to cathode il in the absence of received carrier waves. Moreover, electrons will only pass during that interval in which grids l5 and ii are in phase and are positive. Since the input circuit of phase control grid i3 is connected across inductance 4, it will be apparent that amplitude control grid IE will be of like phase with grid l6 only when amplitude control i5 and its cathode i4 are likewise connected across an inductance, or, in other words, when the path 3, 4 is inductive. This condition occurs only when the incoming carrier waves are higher than the norand-more generally positive.

mal m m. without me ammtion, it should be apparent that device 4 serves to detect and transmit to the primary winding of its transformer 23 as a half cycle impulse the received carrier waves which are of lower frequency than the normal frequency Fa. At the time that thelncoming carrier. wave reaches the normal carrier frequency, the response of discharge device 5, ceases. During the ensuing period when the'incoming carrier waves are of frequency greater than F0, device I delivers to the primary winding of its audio-frequency transformer 24 an impulse constituting the reverse half wave of that previously delivered to the primary winding of transformer 23. From the standpoint of the modulating waves at the transmitting station, discharge device 3 may be considered as transmitting one-half wave and discharge device 6 the reverse half wave. From the'standpoint of received carrier waves" at the receiving station, device 5 may be considered as demodulating those waves of lower than the normal carrier frequency F and device 3 as demodulating those of higher than the carrier frequency F0. Devices and 3, accordingly, op-

erate alternately to supply the opposite half waves of the demodulated signals. Inasmuch as the primary windings of transformers 23 and 24 are differentially connected to the input circuit of audio-frequency amplifier 45, theimpulses which they supply thereto will be additive in the proper sense so that amplifier 45 will supply amplified detected signals to the receiving or loud-speaking device 46. By-pass condenser elements 4'l permit radio frequency components of the output current to readily pass from each cathode to its respective anode and thereby increase the detecting efilciency ofthe electron discharge devices. Series choke coils 25 and 23 exclude the radio frequency current from the primary windings of transformers 23 and 24. Inasmuch as neither electron discharge device 5 nor 6 responds at the normal carrier frequency F0, the circuit will have the advantage of the balanced frequency conversion circuit. permits tuning for any desired normal carrier frequency by variation of the single variable element. Another important feature is that the response characteristics 40 and 4| of the two electron discharge devices are more nearly linear with respect to frequency than are-those of. devices working with the usual high frequency tuned circuit. The output circuit of the two tubes may be simplified by constructing transformers 23 and 24 as a single transformer with two primary windings and a single secondary The polarity and magnitude of the grid bias source 35 which polarizes' phase control grids I2 and I4 may best be found'for the particular tubes employed by experimental determination but,'in v general, the bias potential will be in the neighborhood of zero or a few volts positive or negative A negative potential is, however, conducive to saving of power in the grid circuit and may also be desirable where very high plate potentials are employed.

Where it is not desired to use a low impedance input circuit, such as the path 3, 4, there may be connected between points 38 and 33 the circuit illustrated in Fig. 4 in which the tuned loop comprising inductance 43 and variable capacity. element 50 may be tuned to frequency F1 as indicated in Fig. 5 and the tuned loop comprising inductance SI and variable capacity element 52 It alsomay be tuned to the frequency Fa, as shown in Fig. 5. If the tunings of these two circuits are so chosen that the normal incoming carrier frequency F falls substantially midway between, the device may operate over the range indicated by the portions 53 and 54 oi the graph of. Fig. in substantially the same manner as the circuit of Fig. 1. With this apparatus, the capacity varying elements of devices 56 and 62 are preferably connected mechanically as indicated at 55 to permit a single adjusting operation to tune the circuit.

Fig. 6 illustrates a circuit in which a single electron discharge device 56 is used to effect conversion of a frequency modulated carrier wave to an amplitude modulated carrier wave and at the same time to amplify the converted wave. For that purpose the input path comprising variable condenser 51 and inductance 58 is tuned to a frequency F3 as indicated in Fig. '7. With the amplitude control grid 59 connected through a large stopping condenser 6i across the tuned input path and with the phase control grid 66 connected through high resistance 62 and grid bias source 63 to the junction point 66, it will be obvious that the discharge device 58 will have an operating characteristic somewhat resembling that of. the device 6 of the circuit of Fig. 1 and will accordingly respond to carrier wave frequencies exceeding the cut-off frequency F3 as indicated by the substantially linear graph 65 of Fig. '7. The output current of the device 56 will accordingly be of the same frequency as the input current, but will vary in amplitude with increasing carrier frequency of the input electromotive force, the amplitude being substantially proportional to difference between the incoming frequency and the cut-off frequency F3. It should be noted that the amplitude control grid 59 is biased to the desired potential by a source 66 in series with choke coil 61.

connected to the space current source 69 in the usual manner and serving to prevent electrostatic reaction from the anode 16 of the output circuit from affecting the input electrodes 59 and 60. The output circuit of. the device 56 is connected by a radio frequency transformer I60 to the input circuit ii of an amplitude modulation detecting device 12 of the thermionic type having a grid leak resistance element 13 shunted by the usual grid leak condenser I4. The output circuit of detector 12 comprises a space current path' including source 15 and audio-frequency choke coil 18 and an alternating current path by way of stopping condenser 11 to the input circuit of audio-frequency amplifier 18 in the output circuit of which signal indicating device or loudspeaker 19 is connected.

In some instances, it may be desired in reception of high frequency waves to provide an apparatus having a frequency amplitude conversion effect accompanied with an amplitude limiting action, according to the characteristic of Fig. 8, in which the amplitude response begins at a frequency F4 corresponding to point 8| on the graph and increases linearly with frequency to the frequency F5 corresponding to point8i on the graph and is thereafter limited to a' substantially constant magnitude. Such a characteristic, it will be observed, differs from the characteristic 65 of Fig. "l principally in that it has a limited amplitude portion. It is accordingly possible to utilize an apparatus similar to device 56 of Fig. 6, if means be provided to limit the magnitude of the denser I83 in its input circuit. Included in the device 56 is a shield grid 68 current passed thereby. .As indicated in the schematic diagram of Fig. 9, the apparatus comprises an input circuit having variable tuning condenser 83, inductance coil 86, cathode 85, amplitude control grid 86, phase control grid 81 and anode 88. The phase control grid 81 is connected to the junction point in the tuned input path through a biasing source 89 and a grid leak resistance 96. The amplitude control grid 86 is connected to the outer terminal of condenser 88 through a stopping condenser 9i and a large resistance 92. To neutralize the eflfect of interelectrode capacity between anode 88 and phase control grid 81, a neutralizing path including capacity element 98 of well-known type is connected between the output circuit and the grid 81. Amplitude control grid 86 is biased in the same manner as amplitude control grid 59 of Fig. 6. In operation, the device serves to amplify as does the apparatus 56 of Fig. 6. However, when the amplitude control grid 88 is driven sufliciently positive as occurs during the positive cycle of high intensity oscillations, the drop in potential through series resistance 92 is sumcient to limit the effective potential on grid 86. Inasmuch as the discharge device only passes current during the interval in which the grid 86 is positive, it follows that the maximum amplitude of the amplified oscillations transmitted to the output circuit is effectively limited to a magnitude corresponding to that of the point 82 in the graph of Fig. 8. The output circuit 94 is coupled by an inductance 95 serving as a primary winding to a secondary winding 96 in the circuit 91.

The amplifier 56 of Fig. 6 or the amplifier of Fig. 9 may be connected to the input of a straight line detector where high quality is desired. As shown in Fig. 9, the straight line detector may comprise an' electron discharge device 98 having a series grid resistance 99 with a shunting con- The secondary winding is so designed that the alternating electromotive forces applied to the input circuit of device 98 are of such magnitude as to fall within a range for which the rectified grid current po-- tential is proportional to the input wave envelope. Consequently, there will occur at the grid demodulated or audio-frequency potentials which faithfully represent the modulations of the incoming carrier wave. The output circuit of device 98 is by way of a very large stopping condenser l6l. The space current path includes an audio-frequency choke coil I02 which insures that the demodulated current potentials appearing on the grid are repeated without distortion to the output circuit of the device 98.

What is claimed is:

1. An amplifier for amplifying oscillations offrequency above a fixed cut-off frequency without amplifying oscillations below the cut-oif frequency comprising an electron discharge device having a cathode, an anode and two interposed grids, an inductance element and a capacity ele-.

ment constituting a series path tuned to the cutoff frequency, and means for connecting the cathode and one grid across the inductance element and for connecting the cathode and other grid across the series tuned path.

2. A detector circuit comprising an inductance element and a capacity element connected in a series tuned path and an electron discharge .device having a cathode, two grids and an anode, one of the series tuned path elements being connected between one grid and the cathode, and

the entire series path being connected between the other grid and the cathode.

3. A frequency selective apparatus comprising two paths connected in series, one path having a reactance of positive sign at a predetermined frequency, the other path having a reactance of negative sign at the same predetermined frequency, an electron discharge device having a cathode connected to the free end of one of the paths, an anode, a space current source connected to the cathode and anode, an impedance control element connected to the free end of the second path, and a frequency cut-ofi grid connected to the junction of the two paths.

4. In combination, a receiving circuit for electric, waves, a path comprising two portions in series connected to the receiving circuit, one portion of the path having a definite reactance at a given frequency which is inductive at that frequency and at all lower frequencies, the sec-,

ond portion having an equal reactance at the given frequency which is capacitive at the given frequency and at all higher frequencies, an electron discharge device having a cathode, an anode and two impedance controlling grids, connections each including a polarizing source between the cathode and anode and the cathode and one grid, means connecting the cathode to one terminal of said path, means connecting one of the grids to the opposite terminal of said path, and means connecting the other grid to substantially the junction point between the two series portions of said path.

5. In combination, a receiving circuit for electric waves, a series tuned path comprising an inductance element and a capacity element connected to the receiving circuit, an electron discharge device having a cathode, an anode and two impedance controlling grids, connections each including a polarizing source between the oathode and anode and the cathode and one grid, means connecting the cathode to one terminal of the series tuned path, means connecting one of the grids to the opposite terminal of the series tuned path, and means connecting the other grid to substantially the junction point of the inductance element and the capacity element in the series tuned path.

6. In combination, a receiving circuit for electric waves, a tuned path connected thereto and comprising an inductance element and a capacity element arranged in series, two electron discharge devices each having a cathode, an anode and two impedance controlling grids, an external anode-cathode path including a polarizing source connecting the cathode of each device to its respective anode, means connecting the cathode of one device to one terminal of the tuned path and the cathode .of the second device to the opposite terminal, means connecting one impedance control grid of each device to the terminal of the tuned path opposite the cathode connection of the same device, and means connecting the second impedance control element of each device to the junction point of the inductance element and capacity element in the tuned path.

7; An electron discharge device having a cathode and an anode, a source of space current connected between the cathode and anode, two impedance control elements interposed between said cathode and anode and input paths connecting each of said elements to said cathode, one of said input paths including a source of biasing potential to prevent flow of space current 'in the absence of impressed oscillations, a. circuit connected to both of said paths to impress oscillations there- 'on simultaneously, and one of said paths being tuned to a-' definite frequency whereby the reactance of the path changes sign as impressed oscillations pass through the definite frequency so that for oscillations of a frequency at one side of the definite frequency the impedance control elements are subjected to potentials of opposite phase and for oscillations of a frequency at the other side of the definite frequency the potentials to which the impedance control elements are subjected are in phase agreement.

8. In combination, an electron discharge device having a cathode, an anode and three grids, an input circuit tuned to a definite frequency connected between the cathode and one grid, means for connecting a second one of the grids to an intermediate point in the input circuit which is electrically sufiiciently remote from both the cathode and the first grid so that relatively large alternating electromotive forces at the definite frequency are produced between the point and the cathode and the first grid respectively,

0nd impedance control element within the device connected to an intermediate point which is electrically sufficiently remote from both the cathode and the first impedance control grid so that relatively large alternating electromotive forces are engendered in the operation of the circuit between the point and the cathode and the first impedance control grid respectively in the input circuit, and a space charge grid also within the device and connected directly to a point in the space current source.

10. In combination, an electron discharge device having an effective source of electrons, an anode, an impedance control element associated with the device, an input circuit tuned to a deflnite frequency connected between the source of electrons and the impedance control element, a second impedance control element associated with the device, means for connecting the second impedance control element to a point in the circuit connecting the first impedance control element and the electron source which is electrically sufficiently remote from both the first impedance control element and the source so that in operation a relatively large alternating electromotive force is set up between the point and the first control element and a relatively large alternating electromotive force is set up between the point and the source, an output circuit including a space current source connected between the source of electrons and the anode. and means in the output 

